1. Field of the Invention
The present invention relates to a power supply apparatus which is suitable for use as a half-bridge inverter, an electronic ballast using the power supply, and a self-ballasted fluorescent lamp including the power supply.
2. Description of the Related Art
An electronic ballast for a discharge lamp, including a half-bridge inverter, is known in Japanese Laid Open Patent Application HEI 9-190891 (the ""891 application). According to FIG. 1 of the ""891 application, the ballast comprises a load circuit having an inductance (L2) and at least one of capacitors (C7, C8, C9), a half-bridge inverter including a pair of switching elements (T1, T2), and a drive circuit (AS) generating a drive signal. The drive circuit further comprises a first drive circuit (AS1) and a second drive circuit (AS2) connected to the switching elements respectively. The first drive circuit (AS1) is provided with an additional winding (HW1) and an LC parallel resonance circuit (L3, C3). The second drive circuit (AS2) includes an additional winding (HW2) and a LC parallel resonance circuit (L4, C4). Each additional winding (HW1,HW2) is magnetically to the current-limiting inductance (L2) of the load circuit.
Furthermore, another electronic ballast is disclosed in Japanese Laid Open Patent Application HEI 10-162983 (the ""983 application). According to FIG. 1 of the ""983 application, A half-bridge inverter comprises a resonance load circuit (16) including a resonance inductor (LR), a resonance capacitance (CR), and a pair of complementary switching elements (Q1, Q2). A gate drive circuit (30) of the half-bridge inverter employs a drive inductor (LD) electrically and magnetically coupled to resonance inductor (LR), a secondary inductor (32) connected directly to the drive inductor (LD), and a bi-directional voltage clamp (36) in order to clamp the gate voltage of the pair of switching elements (Q1, Q2). Accordingly, only one gate drive circuit (30) can control a pair of switching elements (Q1, Q2). According to the above prior art ballasts, the additional windings (WH1, WH2) or the drive inductor (LD) are respectively coupled magnetically to the current-limiting inductor (L2) or the resonance inductor (LR).
In these examples, since the current-limiting inductor (L2) or the resonance inductor (LR) has a high resonance voltage during the lamp operation, the additional windings (WH1, WH2) or the drive inductor (LD) should be highly electrically insulated. Otherwise, the additional windings or the drive inductor occasionally operates in error. However, when the additional windings or the drive inductor is highly insulated, the volume of the windings or the drive inductor tends to become larger.
Furthermore, since the current-limiting inductor (L2) or the resonance inductor (LR) has a high resonance voltage during the lamp operation, the temperature rises. As the heat of the inductor (L2, LR) is conducted to the additional windings (WH1, WH2) or the drive inductor (LD), the characteristics of these components tend to change. Therefore, it is difficult to design the additional windings or the drive inductor.
Moreover, the additional windings (WH1, WH2) or the drive inductor (LD) must be exactly located adjacent to the current-limiting inductor (L2) or the resonance inductor (LR). If these components are not accurately arranged relative to the current-limiting inductor (L2) or the resonance inductor (LR), the magnetic characteristics occasionally change. It is difficult to accurately position the current-limiting inductor (L2) and the resonance inductor (LR).
According to one aspect of the invention, a power supply apparatus comprises a direct current power source having a pair of terminals. First and second switching elements are connected in series between the pair of terminals of the direct current power. A load circuit, which includes a resonance inductance and a resonance capacitance, receives a high-frequency alternating current generated by the switching of the first and second switching elements. An unsaturated transformer, has a first winding connected to the load circuit, and a second winding which generates an induced voltage in proportion to the current flowing in the load circuit. A drive resonance circuit includes a capacitance, and the inductance of the second winding. The drive resonance circuit generates a resonance output to cause the first and second switching elements to alternately conduct.
According to another aspect of the invention, an electronic ballast using the power supply comprises the power supply apparatus, and a discharge lamp connected as the load circuit.
According to still another aspect of the invention, a self-ballasted fluorescent lamp comprises the electronic ballast using the power supply, and an enclosure accommodating the discharge lamp.
These and other aspects of the invention will be further described in the following drawings and detailed description of the invention.